Electrochemical machining is a widely used technique for producing holes in difficult-to-machine conductive parts. Generally, this technique involves using electrochemical force (as opposed to mechanical force) to disengage or deplate material from a workpiece.
A highly specialized adaptation of electrochemical machining, known as shaped-tube electrolytic machining, is used for drilling small, deep holes in electrically conductive materials. Shaped-tube electrolytic machining is a noncontact electrochemical-drilling process that distinguishes itself from all other drilling processes by its ability to produce holes with aspect ratios of up to 300:1. Shaped-tube electrolytic machining processes are discussed in more detail in Machining Data Book, vol. 2, pp. 11-71 to 11-75 (3rd ed. 1980); E. J. Weller, Nontraditional Machining Processes, pp. 109-13 (2nd ed. 1984); and G. F. Benedict, Nontraditional Manufacturing Processes, pp. 181-87 (1987).
Advances in jet engine technology have resulted in the need to machine super alloys and metals. The characteristics of these metals and the complex designs associated with jet engine hardware have posed machining problems which are beyond the capability of conventional machining processes. As a result, shaped-tube electrolytic machining processes have found particular applicability in the manufacture of aircraft engines. These processes are especially useful in drilling holes through turbine blades, buckets, vanes, and struts so that cooling liquid can be circulated through these components during turbine operation. Examples of the use of shaped-tube electrolytic machining processes in conjunction with aircraft engine manufacture are disclosed in U.S. Pat. No. 3,352,770 to Crawford et al., U.S. Pat. No. 3,352,958 to Andrews, U.S. Pat. No. 3,793,169 to Joslin, U.S. Pat. No. 3,805,015 to Andrews, and U.S. Pat. No. 4,088,557 to Andrews.
In recent years, shaped-tube electrolytic machining processes have also found application in the manufacture of precision extrusion dies for producing ceramic honeycomb structures. Such structures are particularly useful for automobile catalytic converters.
The manufacture of extrusion dies from these ultra-hard materials is an extremely precise process. The extrusion dies are formed with multiple apertures through which material to be extruded is forced under high pressure. In one method of forming the extrusion die, mechanical drills are used to provide the extrusion apertures. If the extrusion dies are formed of ultra-hard materials such as, for example, 17-4PH stainless steel or Inconel.RTM. 718 (a registered trademark of International Nickel Co., Inc.), the drilling rate used for aperture formation is very slow and a great deal of time and effort is expended in extrusion die formation. If softer die materials are used, the drilling rate is increased, but the life span of the resulting extrusion die is correspondingly shorter.
Because of these difficulties, apertures are now formed in extrusion dies by electrochemical machining techniques rather than mechanical drilling. With an electrochemical machining process, the workpiece from which the die is to be formed is situated in a fixed position relative to a movable manifold. The manifold supports a plurality of drilling tubes, each of which are utilized to form an aperture in the workpiece. The drilling tubes operate as cathodes in the electrochemical machining process, while the workpiece comprises the anode. As the workpiece is flooded with an acid electrolyte from the drilling tubes, material is selectively deplated from the workpiece in the vicinity of the drilling tubes to form the requisite aperture pattern. U.S. Pat. No. 4,687,563 to Hayes and European Patent Application Publication No. 0245 545 to Peters disclose such processes.
A further difficulty in drilling holes in ultrahard materials arises when a series of workpieces is drilled using the same electrolytic fluid. As conductive material deplates from the workpiece and accumulates in the fluid, the fluid's conductivity changes, and, for a given applied voltage in the drilling process, the resultant current flow through the workpiece will vary. As current changes, hole diameter changes accordingly. This problem must be overcome by continuously readjusting input voltage to achieve a uniform hole diameter.
U.S. Pat. No. 3,793,169 to Joslin describes a process for drilling holes of substantially uniform diameter at a high drilling feed rate. To achieve a uniform hole diameter, Joslin teaches slightly reducing supplied current as the depth of the hole increases. Current reduction is accomplished by "programming" a decrease in current as hole depth increases, without regard to actual current flows or conductivity levels. At best, this method for attaining uniform hole diameter is imprecise.
A further problem arises when the workpiece to be drilled includes a slotted lower surface, in the form of a grid of intersecting slots, as is the case in many drilling operations. Holes typically are drilled to meet the intersection of the slots. In such workpieces, resultant hole diameter is reduced as the hole reaches the slotted portion. Such reduced hole diameter is caused by a reduction in the electrolyte back pressure experienced as the holes break into the slotted areas. As the hole being drilled reaches the intersection of slots, electrolyte flows through the slots and back through previously drilled holes, resulting in a loss of electrolyte in the subject hole. This loss of back pressure causes a reduction of the current path, and a consequent reduction of current flow. The reduced current flow slows the deplating rate, which causes the hole diameter to taper where the hole meets the slot. A reduced hole diameter is undesirable at the interface between a hole and slot because it can cause inconsistencies in material flow through a die produced from the workpiece.
The present invention is directed to overcoming these deficiencies.